Carbon nanotubes bonding on metallic electrodes

ABSTRACT

A method for large scale bonding carbon nanotubes on metallic electrodes is disclosed. The method includes that the wafer with a number of patterned electrodes deposited by CNTs via dielectrophoresis method is put onto the pedestal of our designed RF induction heating system. Then, the winding with alternating current induces scroll current on the surface of metallic electrodes due to skin effect. In this case, the current will generate Joule heat which could melt the surface metal of the electrodes. Finally, the CNTs will sink into the surface of the electrodes after short time heating. By varying the parameters of the RF source we can control the bonding process.

FIELD OF THE INVENTION

The invention relates generally to carbon structures. More particularly, the invention relates to carbon nanotubes bonding on metallic electrodes.

BACKGROUND OF THE INVENTION

The present invention relates to a large scale bonding technology of carbon nanotubes (CNTs) and metallic electrodes. And the invention can help realize wafer level CNTs devices fabrication.

The microelectronics industry's interest in the potential use of CNTs as building blocks of nano-scale devices is well founded due to their outstanding electronic, mechanical, and thermal properties. Extensive studies focused on the prototype devices such as field effect transistors and CNTs based sensors were designed and implemented with the objective of a future substitution of current components of digital electronic circuits. In addition, it has also been shown that carbon nanotubes can play a key role as an interconnection for nanoelectronic circuits. Hence, the contact bonding between metal and CNTs becomes an important issue in CNTs based nano-device fabrication.

However, many early nanotube devices just simply deposited CNTs on the top of metal electrodes without reliable bonding contact, which caused weak mechanical performance and high contact resistances between CNTs and electrodes. These disadvantages prevent further practical application of CNTs based device. To achieve better bonding contact between CNTs and metal electrodes, many methods had been utilized. For example, by exposing CNT contact area to a focused electron beam or ion beam, the contact resistance can be decreased by several orders of magnitude.

However, these methods are not easy to realize in real situation due to the limited access to a focused beam. In addition, the rapid thermal annealing method also was demonstrated to be able to reduce contact resistance. In this method, the entire substrate including CNTs, patterned metallic electrodes, and other device components underwent the high temperature process (600-800° C.). Such high temperature will severely limit the selection of materials that can be used on the substrate. Recently, an ultrasonic bonding technique has been developed for bonding single wall carbon nanotubes onto metal microelectrodes.

The bonding was formed by pressing CNTs against the electrodes with a vibrating press at an ultrasonic frequency. Although low resistance contacts between CNTs and metal electrodes can be obtained, the electrodes may be damaged during the bonding process. All these methods have a similar disadvantage from point of view of large scale CNTs devices fabrication for they just bond single device each time. Therefore, in order to realize wide application of CNTs devices, it is necessary to develop large scale bonding technology to satisfy reliable and reproducible large scale CNTs devices fabrication.

SUMMARY OF THE INVENTION

The present invention is to develop a large scale bonding technology of CNTs and metallic electrodes based on RF induction heating method which allows wafer level CNTs devices fabrication due to non-contact operation and local surface heating.

In one embodiment, a wafer with number of patterned electrodes is firstly deposited by CNTs via dielectrophoresis method. Then, the wafer is put on the pedestal of our designed induction heating system. The alternating magnetic field derived from the RF source power induces scroll current on the surface of metallic electrodes due to skin effect. In this case, the current will generate Joule heat which could melt the surface metallic electrodes within very short time, and the extent of melting depends on the power and frequency of our RF source and the heating time.

The shape of the coil and the type of the electrodes also affect the bonding process. Finally, the CNTs will sink into the surface of the electrodes after cooling in vacuum or pretection gas filled environment. As a result, the CNTs on all the electrodes of the wafer are bonded. The invention offers the possibility to realize the wafer level CNTs devices fabrication.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of embodiments and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments and together with the description serve to explain principles of embodiments. Other embodiments and many of the intended advantages of embodiments will be readily appreciated as they become better understood by reference to the following detailed description. The elements of the drawings are not necessarily to scale relative to each other. Like reference numerals designate corresponding similar parts.

FIG. 1 is a system for RF induction heating bonding of CNTs and electrodes.

FIG. 2A is a schematic representation of a wafer including patterned electrodes and CNTs placed in RF induction heating winding

FIG. 2B is a detail schematic representation of electrodes and CNTs patterned on substrate.

FIG. 3A shows an individual carbon nanotube bridges on two isolated electrodes before the RF induction heating bonding.

FIG. 3B shows an individual cabon nanotube sinks into the two isolated electrodes after the radioheating bonding process.

FIG. 4A is a scanning electron micrograph (SEM) of carbon nanotubes and electrodes after the RF induction heating bonding.

FIG. 4B is a SEM of the upsaid electrode in FIG. 4A and the bonded carbon nanotube.

FIG. 4C is a SEM of the blow electrode in FIG. 4A and the bonded carbon nanotubes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following Detailed Description, reference is made to the accompanying drawings, which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. In this regard, directional terminology, such as “top,” “bottom,” “front,” “back,” “leading,” “trailing,” etc., is used with reference to the orientation of the Figure(s) being described. Because components of embodiments can be positioned in a number of different orientations, the directional terminology is used for purposes of illustration and is in no way limiting. It is to be understood that other embodiments may be utilized and structural or logical changes may be made without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of the present invention is defined by the appended claims.

First of all, the electrodes are fabricated by optical lithography and lift-off process. The shape and thickness of the electrodes can vary according to the design of the CNTs device.

Following the fabrication, the CNTs are deposited utilizing dielectrophoresis between pairs of electrodes. The frequency of alternating current in dielectrophoresis is 100 KHz-10 MHz, and the voltage is 1-5V, the time is ranged form 10 second to 30 second. The patterned electrodes and CNTs are shown in FIG. 2B.

FIG. 1 shows the RF induction heating bonding system of CNTs and electrodes. The system contains four parts: RF power 11 offers the power of the heating, water cooling system 12 is to cool the winding 13, and winding 13 is used to induce alternating magnetic field, the glass pedestal 14 supports the wafer during bonding process. The structure of winding can be varied according to electrodes patterns in order to obtain uniform heating.

In the operation, the RF current flow along the winding 13, and generates alternating electric-magnetic field. The glass pedestal 14 is put in the center of the winding 13. The wafer 15 with patterned metallic electrodes and CNTs are put onto the glass pedestal 14, as shown in FIG. 2A. After turning on the power, the alternating electric-magnetic fields generate whirling current on the surface of the metallic electrodes 16 and create heat to melt the surface metal of the electrodes 16. In order to avoid the whole electrodes are melted, the heating time should be not longer than 5 seconds. After natural cooling in vacuum or protection gas filled environment, the CNTs will sink into the electrodes, and the bonding process is finished.

The power of the RF source is ranged from 100 W to 500 W, and the frequency is ranged from 200 KHz to 15 MHz, all these parameters depends on the kinds, shape, size and thickness of the metallic electrodes.

FIG. 3A and FIG. 3B show the state before and after the RF induction heating bonging process. It is clear that the CNTs sink into the electrodes after the bonding, as shown in FIG. 3B.

FIG. 4A is a SEM image of CNTs after the RF induction heating bonding process. The two ends of the CNT sinks into the melting metallic electrode. It is indicated that the CNT is bonded into the electrodes. FIG. 4B shows the upsaid of the electrode shown in FIG. 4A after CNT sinks into it, and FIG. 4C shows the blow of the electrode shown in FIG. 4B after CNT sinks into it.

It is contemplated that features disclosed in this application, as well as those described in the above applications incorporated by reference, can be mixed and matched to suit particular circumstances. Various other modifications and changes will be apparent to those of ordinary skill. 

1. A method for large scale bonding carbon nanotubes on metallic electrodes, comprising: providing a wafer with number of patterned electrodes that is deposited by CNTs via dielectrophoresis method; putting the wafer onto the pedestal of the designed RF induction heating system; turning on the power and heating the wafer for a short time; cooling the wafer in vacuum or protection gas filled environment.
 2. The method according to claim 1, wherein said a RF induction heating system including RF power, water cooling system, winding and glass pedestal.
 3. The method according to claim 2, wherein said the structure of winding can be varied according to electrodes patterns.
 4. The method according to claim 2, wherein said the water cooling system comprises a spiral pipe that coated outsaid of the winding, and the cooling is realized by flowing water through the pipe.
 5. The method according to claim 1, wherein said RF induction heating process comprises tuning the parameters of frequency, power and heating time.
 6. The method according to claim 5, wherein said the frequency of the RF source is ranged from 200 KHz to 15 MHz.
 7. The method according to claim 5, wherein said the power of the RF source is ranged from 100 W to 500 W.
 8. The method according to claim 5, wherein said the time of heating is not longer than 1 minute.
 9. The method according to claim 1, wherein said the CNTs include individual carbon nanotubes, multi-wall carbon nanotubes and bundles of carbon nanotubes.
 10. The method according to claim 1, wherein said the material of electrodes includes metals or alloy.
 11. The method according to claim 10, wherein said the electrodes are fabricated by optical lithography and lift-off process.
 12. The method according to claim 1, wherein said the CNTs are dispersed in aqueous solution.
 13. The method according to claim 1, wherein said the CNTs are bridged between the pairs of electrodes by dielectrophoresis method.
 14. The method according to claim 13, wherein said the frequency of alternating current in dielectrophoresis is 100 KHz-10 MHz.
 15. The method according to claim 13, wherein said the voltage of dielectrophoresis is 1-5V.
 16. The method according to claim 13, wherein said the time of the dielectrophoresis is ranged from 10 second to 30 second. 